The role of TRIB1 in lipid metabolism; from genetics to pathways.

The plasma concentration of lipids is a heritable risk factor for the development of atherosclerosis and related coronary artery diseases (CAD). Mammalian tribbles homologue 1 (TRIB1) is a human locus, the downstream linkage disequilibrium (LD) block of which affects plasma low-density lipoprotein (LDL)-associated cholesterol, triglyceride (TG) levels and CAD across multiple ethnic groups. In addition, association of TRIB1 with non-alcoholic fatty liver disease (NAFLD) has also been shown. A regulatory sequence that enhances TRIB1 promoter activity was identified in the LD block and the minor allele of a single nt polymorphism (SNP, rs6982502) in this regulatory sequence reduces the activity of the TRIB1 promoter. The minor allele of rs6982502 is a risk allele for increasing plasma lipid levels and NAFLD. Trib1 deficiency increases plasma cholesterol and TGs in mice and overexpression of TRIB1 in mouse liver reduces these factors. Expression of rate-limiting lipogenic enzymes is increased in Trib1-knockout mouse liver and decreased with overexpression. Recently, carbohydrate-responsive element-binding protein (ChREBP) emerged as a novel binding partner of TRIB1. Furthermore, novel binding partner, Sin3A (Swi-independent 3A)-associated protein, 18 kDa, was identified, which activates microsomal TG transfer protein (MTTP) expression by binding with MTTP regulatory elements in co-ordination with mSin3A and TRIB1. Very recently, a small molecular compound that up-regulates TRIB1 expression in HepG2 cells has been discovered. Further exploration of the binding partners of TRIB1 and their involvement in lipid metabolism may aid discovery of novel pharmacological targets for the management of dyslipidaemia and steatosis.

Sin3A-associated protein, 18 kDa, a novel binding partner of TRIB1, regulates MTTP expression.

Mammalian tribbles homolog 1 (TRIB1) is a human locus that has been shown to significantly impact plasma lipid levels across several ethnic groups. In addition, the gene has been associated with the occurrence of nonalcoholic fatty liver disease. In the present study, a yeast-two-hybrid system was used to screen for novel molecular targets of TRIB1 binding. Loci corresponding to clones that were positive for TRIB1 binding subsequently were assessed for roles in lipid metabolism in mice using adenoviral constructs to induce knockdown or overexpression. Sin3A-associated protein, 18 kDa (SAP18) was identified as a novel binding partner of TRIB1. Knockdown of the Sap18 in mouse liver decreased plasma lipid levels and increased hepatic lipid levels; SAP18 overexpression showed the opposite effects. Transcriptome analysis of the mouse liver revealed that Sap18 knockdown decreased and SAP18 overexpression increased microsomal TG transfer protein (MTTP) expression levels. Chromatin immunoprecipitation analysis showed that halo-tagged SAP18, halo-tagged TRIB1, and anti-mSin3A antibody enriched precipitates for regulatory sequences of the MTTP gene. Enforced expression of SAP18 enhanced and SAP18 knockdown conversely attenuated the enrichment of MTTP regulatory sequences seen with anti-mSin3A antibody. These studies indicated that SAP18 expression enhanced the recruitment of mSin3A in coordination with TRIB1 to MTTP regulatory elements and increased MTTP expression.

TRIB1 downregulates hepatic lipogenesis and glycogenesis via multiple molecular interactions.

Mammalian tribbles homolog 1 (TRIB1) regulates hepatic lipogenesis and is genetically associated with plasma triglyceride (TG) levels and cholesterol, but the molecular mechanisms remain obscure. We explored these mechanisms in mouse livers transfected with a TRIB1 overexpression, a shRNA template or a control (LacZ) adenovirus vector. The overexpression of TRIB1 reduced, whereas induction of the shRNA template increased, plasma glucose, TG, and cholesterol and simultaneously hepatic TG and glycogen levels. The involvement of TRIB1 in hepatic lipid accumulation was supported by the findings of a human SNP association study. A TRIB1 SNP, rs6982502, was identified in an enhancer sequence, modulated enhancer activity in reporter gene assays, and was significantly (P=9.39 × 10(-7)) associated with ultrasonographically diagnosed non-alcoholic fatty liver disease in a population of 5570 individuals. Transcriptome analyses of mouse livers revealed significant modulation of the gene sets involved in glycogenolysis and lipogenesis. Enforced TRIB1 expression abolished CCAAT/enhancer binding protein A (CEBPA), CEBPB, and MLXIPL proteins, whereas knockdown increased the protein level. Levels of TRIB1 expression simultaneously affected MKK4 (MAP2K4), MEK1 (MAP2K1), and ERK1/2 (MAPK1/3) protein levels and the phosphorylation of JNK, but not of ERK1/2. Pull-down and mammalian two-hybrid analyses revealed novel molecular interaction between TRIB1 and a hepatic lipogenic master regulator, MLXIPL. Co-expression of TRIB1 and CEBPA or MLXIPL reduced their protein levels and proteasome inhibitors attenuated the reduction. These data suggested that the modulation of TRIB1 expression affects hepatic lipogenesis and glycogenesis through multiple molecular interactions.

High prevalence of an anti-hypertriglyceridemic variant of the MLXIPL gene in Central Asia.

MLXIPL is a transcription factor integral to the regulation of glycolysis and lipogenesis in the liver. Common variants of the MLXIPL gene (MLXIPL) are known to influence plasma triglyceride levels in people of European descent. As MLXIPL has a key role in energy storage, genetic variations of the MLXIPL may be relevant to physiological adaptations to nutritional stresses that have occurred during the evolution of modern humans. In the present study, we assessed the phenotypic consequences of the Q241H variant of MLXIPL in populations of Asian and Oceanian origin and also surveyed the prevalence of Q241H variant in populations worldwide. Multiple linear regression models based on 2373 individuals of Asian origin showed that the H allele was significantly associated with decreased concentrations of plasma triglycerides (P=0.0003). Direct genotyping of 1455 individuals from Africa, Asia and Oceania showed that the triglyceride-lowering H allele was found at quite low frequencies (0.00-0.16) in most of the populations examined. The exceptions were some Central Asian populations, including Mongolians, Tibetans and Uyghurs, which exhibited much higher frequencies of the H allele (0.21-0.26). The high prevalence of the H allele in Central Asia implies that the Q241H variant of MLXIPL might have been significant for utilization of carbohydrates and fats in the common ancestors of these populations, who successfully adapted to the environment of Central Asia by relying on nomadic livestock herding.